The present invention relates to a new and improved method of coalescing or interconnecting metallic parts by means of arc fusion welding with a low volume welding seam, generally referred to in the art as narrow gap welds, and also relates to metallic parts produced by such method.
In order to join together thick-wall metallic parts by welding there is presently employed low volume welding seams (narrow gap welding) for economical, thermal and metallurgical reasons. In order to produce such type welding seams there are especially known to the art the use of the shielded-metal-arc-process and electroslag welding. The underlying principles of such welding techniques have been disclosed in numerous publications (see e.g. C. A. Butler, R. P. Meister and M. D. Randall: "Narrow Gap Welding--A Process for All Positions", Weld. Journ. February 1969, pages 102-108; I. D. Henderson, K. Seifert and H. -D. Steffens: "Festigkeits- and Zahigkeitsverhalten von Tiefspaltschweissnahten am Stahl 22NiMoCr 37", DVS-Berichte Nr. 32(1974), pages 321-330; L. E. Stark: "Electroslag Welding with a Consumable Guide Plate and Fiberized Flux", DVS-Berichte Nr. 32(1974), pages 155-159; L. P. Wolff: "Volumenarme Nahtvorbereitung an dickwandigen Bauelementen", DVS-Berichte Nr. 32(1974), pages 217-223).
The shielded-metal-arc-welding process is generally associated with the occurrence of rather pronounced spraying or spattering of metallic particles, sometimes referred to as "spitting" of weld metal, at the region of the arc/shielding gas/molten bath. Spattering can be influenced within certain limits by the nature of the arc, the shielding or protective gas and the composition of the welding wire as well as the base metal, but is never completely avoidable. The resulting spatters contaminate the welding wire-feeding tube, the shielding or protective gas nozzle and the workpiece flanks or sides of the welding groove. Furthermore, the oxidation products of the molten bath upon the surface of the welding beads form fixedly adhering glass-like slag deposits which can only be removed with extreme difficulty and preclude continuous welding in multiple layers. Consequently, the economics of the process are impaired. Contaminated wire feed elements, in turn, produce poor contact of the current infeed and irregular wire feed. Shielding gas nozzles which are coated with spatters lead to the formation of vortexes and disturbances in the shielding gas atmosphere at the region of the molten bath. Spatters at the workpiece flanks of the welding groove produce inclusions and a faulty joint between the base metal and the weld. Finally, slag deposits upon the welding beads lead to inclusions between the individual layers. With this technique it is only possible to produce qualitatively highgrade welding seams by continuously cleaning and removing the spatters and slag.
With electroslag welding there is utilized, in conventional manner, the heat produced by the electrical resistance in a conductive slag bath for melting the base metal and filler material. In order to initiate this process it is however necessary at the start of the welding operation to initially strike an arc and to melt a sufficient quantity of slag forming agents and to bring such into a conductive state. On the other hand, at the end of the welding operation the slag bath or blanket over the molten bath must be maintained by artificial means in order to completely seal the welding seam. The course of this process as a function of time as well as the geometry of these starting and terminal procedures therefore require the application of starting plates and outflow plates, which must be mounted in zones where they can be mechanically removed after or during welding. This constitutes a decisive limitation for this welding process and predominantly in the case of circular seams requires finishing and touch-up work with the aid of other welding techniques. Each interruption of the welding operation multiplies the previously mentioned difficulties, since each time restarting of the welding operation must be carried out. Additionally, complete melting of the slag forming agents and the base metal is oftentimes not possible, leading to a reduction in the quality of the welding seam. The electroslag-welding technique works with comparatively low heating-up and cooling speeds of the filler metal as well as the neighboring thermally-influenced zones of the base metal. This produces a relatively coarse grain structure at the region of the welding seam. In order to overcome the partially poorer mechanical properties in relation to the base metal, there is necessary a subsequent normalizing or tempering of the finished welded work or workpieces. Especially in the case of complicated workpieces this leads to distortion, which only can be eliminated through expensive straightening work.
Considered from the standpoint of the field of application and the products produced, there exists the need for fabrication of large structural components which are as operationally reliable as possible, and which, upon exceeding a predetermined threshold volume and advantageously for metallurgical reasons and with respect to material quality, can be welded together from smaller forged or cast parts. In particular, there exists the need to fay large volume rotational bodies in the heavy duty machine industry from individual parts which are more accessible for material testing, less prone to disturbances in operation, can be controlled better from the standpoint of metallurgical considerations, and to weld such into monolithic workpieces. Such applications require welding seams of the highest quality with concomitant extreme production economies, in order to be successful in relation to other fabrication techniques.